using System;
using System.Collections.Generic;
public class Program
{
public static void Main(string[] args)
string[] data = { "1", "2", "3", "5", "6", "7", "4" };
var list = new List<string>(data);
for (var i = 0; i < list.Count; i++)
if (list[i] == "5")
list.Insert(i, "");
i++;
}
data = list.ToArray();
Console.WriteLine(string.Join(",", data));